Officials on the ground described children eating animal feed and boiled leaves as they took shelter from airstrikes.

Video:Syria: Millions Killed And Injured

The endemic levels of malnutrition have left children "wandering around in a daze from hunger".

Save The Children's chief executive, Tanya Steele, has said Syria's young are "paying the price for the world's inaction" as the fifth anniversary of the civil war fast approaches next week.

Although a cessation of hostilities was brokered last month, allowing more aid to get through, vulnerable families in besieged areas are receiving "only a fraction of what is needed".

One of the 125 adults and children interviewed by the charity compared their plight to living in an "open-air prison" - with women dying in childbirth and babies dying at checkpoints.

The report adds: "They and their families are cut off from the outside world, surrounded by warring groups that illegally use siege against civilians as a weapon of war - preventing food, medicine, fuel and other vital supplies from entering and stopping people from fleeing."

Ms Steele is calling for rival factions to lift their sieges immediately so food and medicine can reach impoverished areas, and for civilians to be granted freedom of movement without fear of gunfire.

She also expressed fears that aid is being used as a "bargaining chip in political negotiations" and insisted humanitarian programmes should not be a factor in any ceasefire discussions.